Nowadays, the application of a multilayer film having a multi-layered structure in which a plurality of kinds of sheet materials are laminated in the thickness direction of the sheet has been spreading to the optical application. As for the multilayer film to be used for the optical application, optical characteristics of the multilayer film are determined by the layer thickness distribution of each sheet material. Therefore, a very high lamination precision is required. Here, the multilayer film is obtained from a laminated sheet formed by laminating a plurality of kinds of sheet materials, and thus the lamination precision of the multilayer film is largely dependent on the lamination precision of the laminated sheet.
A known example of the apparatus and the process for producing a laminated sheet includes the method of producing a laminated sheet including the steps of supplying a plurality of kinds (particularly two kinds) of sheet materials (typically a melting resin, etc.) to each manifold, diverting the sheet material supplied to each manifold through multiple slits to form a laminate with multiple layers, and discharging the laminate from a die having a slit gap extending in the width direction of the laminated sheet (hereinafter referred to as width direction). Then, the laminated sheet discharged from the die is solidified and left or subjected to post-treatment such as drawing to be formed into a multilayer film (hereinafter, product obtained by solidifying laminated sheet is referred to as multilayer film).
Subsequently, a typical example of the laminated sheet producing apparatus is described. FIG. 1 is a perspective view for explaining a typical laminated sheet production apparatus and a production process. As shown in FIG. 1, the laminated sheet producing apparatus includes sheet material introducing pipes 1 and 2 which supply sheet materials A and B, a multilayer laminating apparatus 3 which forms a laminate in which a large number of the sheet materials A and B supplied by the sheet material introducing pipes 1 and 2 are alternately laminated, a conduit 4 which guides the laminate to the downstream, a die which adjusts the width and thickness of the laminate from the conduit 4 to a predetermined value, discharges the adjusted laminate, and forms a laminated sheet, and a casting drum 7 which cools and solidifies a laminated sheet 6 discharged from the die 5. The laminated sheet solidified by the casting drum 7 is usually sent to a drawing process (not shown) as indicated by an arrow NS and drawn in one or two directions to form a multilayer film.
Here, a typical example of the multilayer laminating apparatus 3 is described. FIG. 2 is a partial perspective view showing only an internal space allowing sheet materials or the like for a typical multilayer laminating apparatus to pass through. As shown in FIG. 2, the multilayer laminating apparatus 3 includes sheet material introducing channels 21 and 22 which supply the sheet materials A and B, manifolds 23 and 24 which uniformly distribute the sheet materials A and B supplied from the sheet material introducing channels 21 and 22 to the lamination direction of the laminated sheet (hereinafter referred to as lamination direction), lines of a large number of pores 25 and 26 which separate the sheet materials A and B from the manifolds 23 and 24 into a predetermined layer number, lines of a large number of slits 27 and 28 which guide the sheet materials A and B from each of the pores 25 and 26 to the downstream, and a lamination completing unit 29 which forms a laminate in which a large number of the sheet material A and B from each of the slits 27 and 28 are alternately laminated.
However, according to a finding of the present inventors, when a laminated sheet for optical application is formed in which a large number of two kinds of sheet materials (sheet materials A and B) are alternately laminated using the conventional laminated sheet producing apparatus, it has been found that a multilayer film in which the thickness of each layer is not uniform in the width direction is obtained from the formed laminated sheet by rapid flow change due to widening in the width direction of the die 5. FIG. 9 is a cross-sectional view of a multilayer film produced using the conventional laminated sheet producing apparatus. As shown in FIG. 9, a multilayer film 40 includes a layer 41 of the sheet material A and a layer 42 of the sheet material B. When the cross section of the multilayer film 40 is observed, it is found that the thickness of the layer closer to the surface layer tends to be thinner in the layers 41 and 42 at a central portion in the width direction and the thickness of the layer closer to the surface layer tends to be thicker at an end portion in the width direction. That is, the thickness of each layer is not uniform in the width direction and the lamination precision is low. Therefore, in the case of the conventional laminated sheet producing apparatus, it has sometimes been difficult to form a lamination sheet which sufficiently satisfies the lamination precision required for a multilayer film to be used for optical application.
Under such circumstances, the present applicants disclose a technique of the multilayer laminating apparatus 3 in which the thickness of each layer becomes uniform in the width direction in Patent document 1. FIG. 3 is a partial perspective view showing only an internal space allowing sheet materials or the like for a multilayer laminating apparatus which is used in the laminated sheet producing apparatus proposed by the applicants of Patent document 1 to pass through. The basic structure of the multilayer laminating apparatus 3 shown in FIG. 3 is almost the same as that of the multilayer laminating apparatus 3 shown in FIG. 2. However, they are different from each other in that there is nothing corresponding to the pores 25 and 26 of FIG. 2 and the upper portions of the slits 27 and 28 are obliquely formed. The unevenness of the flow rate in the width direction of the sheet material is resolved by obliquely forming the upper portions of the slits in the multilayer laminating apparatus 3 shown in FIG. 3 and the thickness of each layer may become uniform in the width direction. However, the lamination precision is further required in the optical application, which requires the lamination precision strictly.
According to a finding of the present inventors, the following reason is considered as a cause of the phenomenon as shown in FIG. 9. When a fluid flows through a channel, the flow rate distribution of the fluid varies depending on a cross-sectional configuration perpendicular to the flow channel direction. That is, when the cross-sectional configuration perpendicular to the flow channel direction is changed to the flow channel direction, the flow of a fluid is generated in a direction perpendicular to the flow channel direction of the flow channel (namely, width direction or lamination direction). When the typical laminated sheet producing apparatus of FIG. 1 is taken for example, the die is rapidly widened in the width direction and then reduced in the lamination direction. Thus, the cross-sectional configuration of the flow channel is rapidly changed by the die and the portion near the surface layer of the laminate flows toward both end portions in the width direction. The surface layer at the central portion in width direction is thinned by the flow of the laminate thus generated and the thickness of each layer becomes non-uniform in the width direction. Therefore, it is considered that the lamination precision becomes low.
Incidentally, an example of a technique which looks similar to the preferred embodiments of the present invention at the first glance includes a laminated sheet producing apparatus disclosed in Patent document 2.
A method for reducing the cost by using an inexpensive resin for both end portions of a laminated sheet in the width direction is disclosed in Patent document 2. FIG. 18 is a cross-sectional view perpendicular to the width direction of the multi-manifold die of Patent document 2. FIG. 19 is a cross section of a line H-H of FIG. 18, namely, a cross-sectional view perpendicular to the lamination direction of the multi-manifold die of Patent document 2. As shown in FIGS. 18 and 19, a multi-manifold die 70 includes resin flow channels 71 to 73 which supply resins A, B, and C, manifolds 74 to 76 which uniformly distribute the resins A, B, and C supplied from the resin flow channels 71 to 73 in the width direction, slits 77 to 79 which guide the resins A, B, and C from the manifolds 74 to 76 to the downstream, a lamination completing unit 80 which laminates the resins A, B, and C from the slits 77 to 79 to form a laminate, a resin flow channel 81 which diverts the resin A flowing through the resin flow channel 71, a confluence portion 82 which adds the resin A from the resin flow channel 81 to both end portions in the width direction of the laminate as an edge portion, and a slit 83 which discharges the laminate formed by the confluence portion 82 to form a laminated sheet. The formation of the laminated sheet using the multi-manifold die 70 allows only the edge portion formed solely by the resin A which is inexpensive to be cut in the following step. The amount of the expensive resins B and C used and the cost may be reduced.
However, according to a finding of the present inventors, the conventional laminated sheet producing apparatus described in Patent document 2 is a technique aiming at cost reduction at the time of production. Since a technical idea in which the thickness of each layer is uniform to the width direction is not disclosed, it does not reach the level which survives the optical application at a point of the lamination precision. This is because such a technique is aimed at controlling a layer whose layer number is several, namely, the layer with a thickness of several tens μm per layer. Therefore, there is no problem to control a layer whose layer number is several hundred, namely, the layer with a thickness of several tens nm per layer like a film for optical application.    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2006-123541    Patent document 2: JP-A No. 6-91719